Im not wrong, I know quite a bit about NFA, and I offered no advice.
http://www.atf.gov/firearms/guides/identification-of-nfa-firearms.html
Search the page for “Wallet Holsters” and you’ll learn that the illustrated image is an AOW requiring registration and a $200 tax. Granted, the holster without the gun isn’t regulated, but putting a gun in it is.
I am aware that there is such a thing as a wallet holster that the ATF describes as a “any other weapons”. It’s just that the holster pictured on the thread is not one of those. If a holster does not “hide” the gun, or make it appear to be something other than the gun, the ATF has “ruled” that a shoot through holster in and of itself does not make it a “any other weapon”. With the ATF this might change tomorrow.